The invention relates to a screw having at least one thread that is formed by means of a rolling process, especially a flat-die rolling process, whereby the thread consists of two ridges of material which are shaped from a blank by means of cold-forming during the rolling process in such a way that the thread has a closing crease where the ridges of material meet each other. The invention also relates to a method for forming a screw thread on the lateral surface of an essentially cylindrical blank so as to form at least one thread, by means of rolling, especially flat-die rolling.
The invention especially relates to self-cutting screws.
So-called flat-die rolling processes are known for purposes of forming threads on screws. In this process, an essentially cylindrical blank that is to be machined is positioned between two profiled flat dies which are then moved linearly towards each other. The profiling on the flat dies then forms a thread on the lateral surface of the blank. During the rolling process, the blank is rotated between the flat dies, so that at least one continuous thread is formed circumferentially on the lateral surface. The thread is normally formed in that the profiling of the flat dies causes two ridges of material to be pressed out of the screw, and these ridges are then united during the rolling process to form a thread. A so-called closing crease is formed at the transition site between the ridges of material. The thread, however, is shaped by cold-forming, that is to say, the two ridges of material cannot be completely joined to each other by adhesive force. In other words, even though the two ridges of material are pressed against each other with a positive closure, a gap sometimes remains between these ridges, starting at the closing crease and constituting a weak spot in the material. In the prior-art methods, the closing crease is situated here precisely in the vertex of the thread.